Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 5,127.0	$ 4,787.0	[1]
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	500.0	489.8
Fair Value, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	5,032.9	4,411.6
Fair Value, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,745.6	3,460.4
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	223.6	223.7
Fair Value, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	328.3	236.2
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	264.5	216.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	160.1	239.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,861.8	1,757.5
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	219.0	202.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	521.3	410.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	167.0	172.8
Fair Value, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	400.6	354.5
Fair Value, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	405.9	114.4
Fair Value, Recurring [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	480.8	482.3
Fair Value, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,165.6	1,026.9
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	324.2	226.7
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	323.0	226.7
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1.2	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	392.1	346.3
Fair Value, Recurring [Member] | Level 1 [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	449.3	453.9
Fair Value, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,852.0	3,374.3
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,414.6	3,231.5
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	223.6	223.7
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	5.3	9.5
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	264.5	216.8
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	160.1	239.8
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,853.8	1,755.3
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	219.0	202.8
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	521.3	410.8
Fair Value, Recurring [Member] | Level 2 [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	167.0	172.8
Fair Value, Recurring [Member] | Level 2 [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value		0.0
Fair Value, Recurring [Member] | Level 2 [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	405.9	114.4
Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	31.5	28.4
Fair Value, Recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	15.3	10.4
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	6.8	2.2
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	6.8	2.2
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	8.5	8.2
Fair Value, Recurring [Member] | Level 3 [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
Fair Value, Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 0.0	$ 0.0

[1] Derived from audited consolidated financial statements.